Because his work is relevant to the historical context in which science has progressed. It is not uncommon to discuss the work of someone in the past who has made a contribution toward modern conceptions of scientific discoveries because the conclusions or outcomes that an investigator reaches is only part of what may be learned. In psychology, Gall is often used (along with Freud and others) to show the pitfalls of making certain (often flawed) assumptions while arguably maintaining scientific rigor throughout their investigations. Other examples highlight the reverse, but all have been used to glean valuable lessons; some less obvious than others.
